Gas-to-gas heat exchanger

ABSTRACT

In a gas-to-gas heat exchanger, numerous tubes extend in parallel between a pair of tubesheets that are oblique to one another. Each tube is medially curved to have its end portions normal to the respective tubesheets to which they connect, all being curved in the same direction. The medial portions of the tubes extend through a baffle disposed to bisect the acute angle between the tubesheets. Side and end walls cooperate with the baffle and tubesheets to define two cooling air passage portions, at opposite sides of the baffle, and a third connecting them around the baffle.

United States Patent Csathy [$4] GAS-TO-GAS HEAT EXCHANGER [72] Inventor: Denh G. Csathy, Minneapolis,

Minn.

[73] Assignee: RayGo, Inc., Minneapolis, Minn.

[22] Filed: Aug. 23, 1971 [2!] Appl. No.: 173,820

[52] [1.8. CI ..l65/l59 [51] Int. Cl. ..F28f 9/22 [58] Field of Search ..l65/l57-159, 8l-83 {56] References Cited UNITED STATES PATENTS l,745,093 1/1930 Heimbergor [65/82 3,118,497 1/1964 Olson ..l65/l57 3,495,556 2/1970 Steever et al. ..l65l82 X 1 Nov. 14, 1972 Primary Examiner-Milton Kaufman Assistant Examiner-T. W. Streulc, .lr. Attorney-Ira Milton Jones ABSTRACT In a gas-to-gas heat exchanger, numerous tubes extend in parallel between a pair of tubesheets that are oblique to one another. Each tube is medially curved to have its end portions normal to the respective tubesheets to which they connect, all being curved in the same direction. The medial portions of the tubes extend through a bafiie disposed to bisect the acute angle between the tubesheets. Side and end walls cooperate with the baffle and tubesheets to define two cooling air passage portions, at opposite sides of the baffle, and a third connecting them around the baffle.

4 Claim, 2 Drawing Figures GAS-TO-GAS HEAT EXCHANGER This invention relates to gas-to-gas heat exchangers and is more particularly concerned with a heat exchanger that is well adapted for cooling a gas that carries a large amount of fine particulate material such as carbon black.

Carbon black is produced in a reactor in which a high temperature continuous process takes place. The carbon black is carried out of the reactor entrained in a stream of gas that has a temperature which may be in excess of 2,000F. Before the carbon black can be separated from the gas, the mixture must be substantially cooled, and this is done by passing it through a heat exchanger almost immediately upon its leaving the reactor.

In the past, the cooling medium in the heat exchanger was usually water. However, it has been recognized for some time that production economy can be materially improved if the hot gas from the reactor is made to give up its heat to air, and the air thus heated is then utilized as combustion air for the reactor burners. Such preheating of the combustion air has the effect of returning to the reactor a substantial portion of the heat that was carried away from it in the gas, thereby reducing the amount of fuel that must be burned in the reactor per pound of carbon black produced.

The provision of a gas-to-gas heat exchanger that would be satisfactory under the conditions encountered in carbon black production has heretofore presented serious problems. In a gas-to-gas heat exchanger the hot gas to be cooled is conducted through a number of tubes that are connected in parallel, and the air or other gaseous cooling medium is circulated across the exterior of these tubes. The tubes are secured in laterally spaced relation to one another by means of a pair of tubesheets to which the opposite end portions of each tube are respectively connected. One of the tubesheets can of course cooperate with other means to provide an inlet chamber from which the hot gas enters the tubes, the other can comprise a part of an outlet chamber into which the cooled gas flows as it leaves the tubes.

When the gas to be cooled carries an entrained finely divided material such as carbon black, it is almost inevitable that from time to time one or another of the tubes will become plugged with an accumulation of such material. When this happens, there is of course no flow of gas through the plugged tube, and its temperature falls substantially below that of the remaining tubes. Because of the high temperatures to which the heat exchanger is subjected, its tube are often made of a high alloy metal, such as stainless steel, that has a relatively high coefficient of thermal expansion. Hence the drop in temperature that occurs upon plugging of a tube causes a substantial change in the length of the plugged tube relative to the unplugged ones, with the result that severe stresses can be imposed upon both the tubes themselves and their connections with the tubesheets.

With the foregoing in mind, it is one of the objects of the present invention to provide a gasto-gas heat exchanger that is especially well adapted for cooling a gas in which finely divided particles are entrained, by reason of the fact that if any tube in the heat exchanger should be plugged, any change in its length relative to the remaining tubes is accommodated in the plugged tube itself without exerting substantially high forces upon the tubesheets and its connection thereto.

Another and very important object of this invention is to provide a gas-to-gas heat exchanger which is well adapted for the cooling of gas in which particulate material is entrained and which is very compact in relation to its cooling capacity and, further, can be installed in any orientation, to afford great economy and efficiency in the utilization of space in a plant in which it is installed.

Another object of this invention is to provide a gasto-gas heat exchanger through which the gas to be cooled can flow without undergoing any substantial or abrupt changes in flow direction, so that such gas will not tend to drop or deposit any entrained particulate material that it carries, and in which there is a good circulation of cooling medium over the tubesheets to assure that they will not become overheated.

With these observations and objectives in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings which exemplify the invention, it being understood that such changes in the specific apparatus disclosed herein may be made as come within the scope of the appended claims.

The accompanying drawings illustrate one complete example of the embodiment of the invention constructed according to the best mode so far devised for the practical application of the principles thereof, and in which:

FIG. 1 is an end view of a gas-to-gas heat exchanger embodying the principles of this invention; and

FIG. 2 is a sectional view through FIG. 1 on the plane of the line 2-2.

Referring now more particularly to the accompanying drawings, the numeral 5 designates generally a gasto-gas heat exchanger which embodies the principles of this invention and which comprises, in general, a plurality of tubes 6, a pair of tube-sheets 7 and 8, and a baffle 9.

The tubesheets 7 and 8 hold the tubes 6 in laterally spaced apart relationship and can also comprise parts of chambers 10 and 11 with which the tubes communicate at their opposite ends. Thus the tubesheet 7 can comprise one wall of an inlet chamber 10 from which gases to be cooled flow into the tubes, and the tubesheet 8 can define a wall of a collecting or outlet chamber 11 into which the gases flow as they leave the tubes It will be understood that the tubes are connected in parallel, so that the gas to be cooled normally flows from the chamber 10 to the chamber 11 through all of the tubes simultaneously.

The tubesheets and the baffle 9 are secured in fixed relation to one another by means of opposite side walls 12. In turn, these side walls, the tubesheets and the baffle cooperate with one another and with a rear wall 13 to define a duct or passage 14 through which a gaseous cooling medium can flow and across which the tubes 6 extend to provide for exchange of heat between the gases flowing inside the tubes and the cooling medium circulated around their exteriors. Since air is the most common gaseous cooling medium, the description will proceed on the basis that air is to be circulated through the passage 14, and it will be understood that the air can be forced through that passage by means of a fan or blower (not shown) and that upon leaving the heat exchanger it may be conducted to a reactor or other point of use (not shown) at which the heat that it has gained in the heat exchanger can be advantageously utilized.

The two tubesheets 7 and 8 can be identical. Each can comprise a flat rectangular metal plate having holes at more or less regularly spaced intervals. Each of the holes is of course intended to receive an end portion of one of the tubes 6, and the tube has a gas-tight securement to the tubesheet, as by means of a weldment 16 all around the tube.

It is a feature of the heat exchanger of this invention that the tubesheets are disposed obliquely to one another, with their front edges 17 and rear edges 18 parallel, and with their rear edges nearer to one another than their front edges but spaced apart by a substantial distance. Stated another way, the tubesheets lie in planes which are at an acute angle to one another and which intersect at some distance behind the rear edges of the tubesheets.

Each of the tubes 6 has a curved medial portion, as at 19, so that the end portions of the tube (which are straight) have their axes at an obtuse angle to one another that is complementary to the acute angle between the planes of the tubesheets. Each end portion of every tube is of course normal to its adjacent tube sheet, so that all of the tubes have their medial portions curved through the same angle and in the same direction. The tubes are arranged in a bank of side-bysidc rows of identical sized and shaped tubes, but the tubes of the several rows are progressively longer, the shortest tubes being at the inside of the angle formed by the end portions of the rearmost row of tubes.

The curve 19 in each tube permits the tube to have a certain amount of lengthwise expansion or contraction relative to its neighbors without disturbance to the connections between the tubes and the tubesheets, such relative expansion or contraction of the tube being accommodated by a flexing of the tube to change the radius of curvature of its medial portion.

The tubes are provided are their exteriors with fins 20. It is appreciated that the fins do not greatly increase the efficiency of heat transfer through the tubes, inasmuch as the film coefficients of the gas and the cooling air do not differ greatly; but the fins are not intended to afford compactness to the heat exchanger. Rather, the purpose of the fins is to maintain the tube metal at a low enough temperature to allow the tubes to be made of relatively inexpensive metal without danger of their being overheated. Thus, the tubes and fins can be made of carbon steel even if the gas enters the heat exchanger at temperatures as high as 1,400 to l ,600F., providing the cooling air has a reasonably low temperature at its inlet. At higher inlet temperatures of the gas, of the cooling air, or of both, up to gas inlet temperatures of about 2,200F., stainless steel tubes with stainless steel fins would be preferred.

Since the temperature of the tube metal is thus kept relatively low in the heat exchanger of this invention, this feature cooperates with the curved medial portions 19 of the tubes to eliminate almost completely the pos sibility of a tube joint failure in case one of the tubes becomes plugged, inasmuch as the temperature difference and hence the thermal stress will be low as between the plugged and unplugged tubes. Furthermore, since the tubes can be of relatively large diameter because of the presence of the cooling tins, and they have only a moderate curvature at their medial portions, there is little likelihood of the tubes being plugged.

The baffle 9 can comprise a flat, rectangular steel plate having the same width as the tube-sheets. It is also similar to the tubesheets in having a pattern of holes to accommodate the tubes, which have their curved medial portions extending through the baffle. Preferably the holes in the baffle fit the tubes snugly enough to provide for heat transfer between the tubes and the baffle, so that the baffle tends to equalize the metal temperature of the tubes, further minimizing thermal stresses on the connections between the tubes and the tubesheets.

The baffle lies in a plane that bisects the acute angle between the planes containing the tube-sheets. The front and rear edges of the baffle are parallel to the front and rear edges of the tubesheets, and the side edges of the baffle lie in common planes with the respective side edges of the tubesheets so that the side walls 12 can comprise flat plates that cooperate with the bafile and the tubesheets to secure the bafile and tubesheets in fixed relation to one another. Attention is directed to the fact that the rear edge of the baffle is adjacent to the curved medial portion of the shortest row of tubes, and its front edge adjacent to the curved medial portion of the longest row of tubes. Stay rods 21 tie the side walls together and by being engaged along their lengths with the opposite surfaces of the baffle, serve to hold the baffle in place as well as stiffening the side walls.

The side walls 12 also cooperate with the baffle and tubesheets to define two portions 22 and 23 of the cooling air passage 14, one at each side of the baffle. Note that each of these passage portions is defined in part by one of the tubesheets as well as by a surface of the baffle. Another air passage portion 24, which communicates the passage portions 22 and 23 with one another around the baffle, is conjointly defined by the side walls 12 and the rear wall 13.

The rear wall is connected with the rear edges 18 of the tubesheets and extends from one to the other of them in a curve of substantially uniform radius that merges smoothly and tangentially into the tubesheets at its zones of connection with them. The rear wall is spaced a substantial distance from the rear edge of the baffle, and hence it effects a smooth change of flow direction in the air moving through the passage 14, guiding it through almost a complete reversal of direction in conducting it from one to the other of the passage portions 22 and 23.

A pair of front wall portions 25 extends between the battle and each of the tubesheets, as well as from one to the other of the side walls. These front wall portions are preferably arcuate in shape and each has in it a port, the port 26 in one of them being an air inlet and the port 27 in the other being an air outlet. A flange 28 around each of these ports provides for sealed connection with suitable air inlet and outlet ducting (not shown).

Preferably the directions of air and gas flow through the heat exchanger are such as to provide for counterflow, that is, cooling air enters the air passage 14 through the port 26 and first traverses the hottest parts of the tubes, in the passage portion 22, before crossing the cooler portions of the tubes in flowing through the passage portion 23, and thence emerges through the port 27.

In flowing through the passage portions 22 and 23 the airstream sweeps the tubesheets 7 and 8 respectively, and the curved path that it must follow through the passage portion 24 imparts a certain amount of centrifugal force to the air that insures its impingement against those tubesheets. The cooling air thus maintains the tubesheets at a low enough temperature to prevent their being deteriorated by overheating.

To further protect the tubesheets from overheating, each has a layer of refractory insulating material 30 overlying its surface remote from the air passage 14. A refractory that has been found suitable for this purpose is a relatively hard and dense material sold under the trademark Greencast-94". To provide for communication with the tubes through the refractory insulation, a ferrule 31 projects through the refractory and a short distance into each end portion of each tube. Since the ferrules are exposed to relatively high heat, they are preferably of stainless steel. Each ferrule must be anchored to its tube, but to prevent any substantial heat transfer between the ferrule and the tube the ferrule is merely tack welded to the tube at only two diametrically opposite spots.

The preferred orientation of the heat exchanger of this invention is with the rows of tubes 6 lying in vertical planes, and with the inlet chamber uppermost as shown in FIG. 2. With that orientation, gravity supplements the effects of the gas stream in moving particulate material through the tubes and thus tends to prevent plugging. However, the unit could be installed with its tubes disposed in horizontal planes, and will even operate satisfactorily with an upward flow through the tubes 6 should installation requirements make this necessary.

From the foregoing description taken with the accompanying drawings it will be apparent that this invention provides a gas-to-gas heat exchanger that is especially suitable for cooling a gas that carries an entrained particulate material such as carbon black, inasmuch as the tubes and tubesheets in the heat exchanger of this invention are not subjected to severe thermal stresses in the event one of the tubes becomes plugged. It will also be apparent that there is little tendency toward tube plugging in the heat exchanger of this invention because the gas to be cooled does not undergo substantial changes in flow direction in the course of flow through it, but that it is nevertheless unusually compact relative to its capacity.

Those skilled in the art will appreciate that the invention can be embodied in forms other than as herein disclosed for purposes of illustration.

The invention is defined by the following claims:

1. A gas-to-gas heat exchanger for cooling a hot gas and which is of the type comprising a plurality of tubes through which gas to be cooled can be caused to flow and across which gaseous cooling medium is conducted, and a pair of tubesheets, each connected with an end of all of the tubes and which cooperate to hold the tubes in laterally spaced apart relation to one another, one of said tubesheets comprising a pan of a hot gas inlet chamber that opens into one end of every tube and the other comprising a part of a cooled gas outlet chamber into which the other end of every tube opens, said heat exchanger being characterized by:

A. each of the tubes having its medial portion curved so that its opposite end portions have their axes at an obtuse angle to one another, all of the tubes being curved in the same direction and through substantially the same angle, and said plurality of tubes being arranged in a bank of progressively longer tubes;

B. each tubesheet being substantially normal to its adjacent end portions of the tubes, so that the tubesheets lie in planes that are at an acute angle to one another;

C. a baffle through which the curved medial portions of the tubes extend and which lies in a plane that substantially bisects said acute angle defined by the planes of the tubesheets, said baffle having a read edge adjacent to the curved medial portion of the shortest tubes and a front edge adjacent to the curved medial portion of the longest tubes;

D. side wall means cooperating with the tubesheets and the baffle to define one cooling medium passage between the baffle and one of the tubesheets and another cooling medium passage section between the baffle and the other tubesheet; and

E. curved rear wall means extending between and tangentially joined with the adjacent edges of the tubesheets and also connected to said side wall means but spaced from the inner edge of the baffle to define a passage portion that communicates said passage sections and cooperates with the baffle to compel a substantial change in flow direction of cooling medium flowing from one to the other of said passage sections.

2. The gas-to-gas heat exchanger of claim 1, further characterized by:

F. said tubes having external cooling fins.

3. The gas-to-gas heat exchanger of claim I, further characterized in that the curvature of said curved rear wall is uniform and continuous to the tangent connections thereof with the tubesheets.

4. A gas-to-gas heat exchanger of the type comprising a plurality of tubes through which gas to be cooled can be caused to flow and across which a gaseous cooling medium can be circulated, and a pair of tubesheets which cooperate to hold the tubes in laterally spaced relationship, one of said tubesheets being connected with one end of every tube and being cooperable with other means to direct gas to be cooled into said ends of the tubes, and the other tubesheet being connected with the other end of every tube, said heat exchanger being characterized by:

A. the tubesheets being disposed in planes which are at an acute angle to one another;

B. each tube having its medial portion curved to dispose each of its end portions substantially normal to its adjacent tubesheet, and said plurality of tubes being arranged in a bank of progressively longer tubes;

C. a baffle disposed between the tubesheets in a plane substantially bisecting the acute angle defined by the planes of the tubesheets and through which baffle the curved medial portions of the tubes extend, said baffle having a rear edge adjacent to the curved medial portion of the shortest tubes and a front edge adjacent to the curved medial portion of the longest tubes; and

D. wall means cooperating with the tubesheets and the baffle to define a pair of cooling medium passage portions, one at each side of the baffle, 

1. A gas-to-gas heat exchanger for cooling a hot gas and which is of the type comprising a plurality of tubes through which gas to be cooled can be caused to flow and across which gaseous cooling medium is conducted, and a pair of tubesheets, each connected with an end of all of the tubes and which cooperate to hold the tubes in laterally spaced apart relation to one another, one of said tubesheets comprising a part of a hot gas inlet chamber that opens into one end of every tube and the other comprising a part of a cooled gas outlet chamber into which the other end of every tube opens, said heat exchanger being characterized by: A. each of the tubes having its medial portion curved so that its opposite end portions have their axes at an obtuse angle to one another, all of the tubes being curved in the same direction and through substantially the same angle, and said plurality of tubes being arranged in a bank of progressively longer tubes; B. each tubesheet being substantially normal to its adjacent end portions of the tubes, so that the tubesheets lie in planes that are at an acute angle to one another; C. a baffle through which the curved medial portions of the tubes extend and which lies in a plane that substantially bisects said acute angle defined by the planes of the tubesheets, said baffle having a read edge adjacent to the curved medial portion of the shortest tubes and a front edge adjacent to the curved medial portion of the longest tubes; D. side wall means cooperating with the tubesheets and the baffle to define one cooling medium passage between the baffle and one of the tubesheets and another cooling medium passage section between the baffle and the other tubesheet; and E. curved rear wall means extending between and tangentially joined with the adjacent edges of the tubesheets and also connected to said side wall means but spaced from the inner edge of the baffle to define a passage portion that communicates said passage sections and cooperates with the baffle to compel a substantial change in flow direction of cooling medium flowing from one to the other of said passage sections.
 2. The gas-to-gas heat exchanger of claim 1, further characterized by: F. said tubes having external cooling fins.
 3. The gas-to-gas heat exchanger of claim 1, further characterized in that the curvature of said curved rear wall is uniform and continuous to the tangent connections thereof with the tubesheets.
 4. A gas-to-gas heat exchanger of the type comprising a plurality of tubes through whiCh gas to be cooled can be caused to flow and across which a gaseous cooling medium can be circulated, and a pair of tubesheets which cooperate to hold the tubes in laterally spaced relationship, one of said tubesheets being connected with one end of every tube and being cooperable with other means to direct gas to be cooled into said ends of the tubes, and the other tubesheet being connected with the other end of every tube, said heat exchanger being characterized by: A. the tubesheets being disposed in planes which are at an acute angle to one another; B. each tube having its medial portion curved to dispose each of its end portions substantially normal to its adjacent tubesheet, and said plurality of tubes being arranged in a bank of progressively longer tubes; C. a baffle disposed between the tubesheets in a plane substantially bisecting the acute angle defined by the planes of the tubesheets and through which baffle the curved medial portions of the tubes extend, said baffle having a rear edge adjacent to the curved medial portion of the shortest tubes and a front edge adjacent to the curved medial portion of the longest tubes; and D. wall means cooperating with the tubesheets and the baffle to define a pair of cooling medium passage portions, one at each side of the baffle, said wall means including a wall portion which extends between and is connected with the adjacent edges of the tubesheets and which is in spaced relation to the inner edge of the baffle to provide another passage portion around the inner edge of the baffle to communicate said pair of passage portions with one another and compel a substantial change in flow direction in cooling medium flowing from one to the other of said pair of passage portions. 